Factors to be Considered in Selection of Fuse:
In order to ensure that the fuses will correctly and reliably protect any given section or element of an electrical circuit, selection of fuse characteristics and ratings must be suitable in accordance with the following requirements.
Rated Voltage : Fuses and their fusible elements must be selected so that their voltage rating is the same as the service voltage of the circuit they are to be inserted in.
Maximum Current Rupturing Capacity : To extinguish the arc resulting from blowing of the element in a fuse safely, the latter and its element must have a current interrupting capacity at least equal to or greater than the largest short-circuit current which can flow in the circuit to be protected.
Rated Current : The fuse element to be used in a fuse must be selected for a rated current large enough to ensure that the element will not fuse when the circuit is carrying its sustained maximum load current and when the circuit is carrying a short-time overload current.
To satisfy the first of these requirements the following condition must be observed to continue normal operation when practically a continuous maximum load current is being carried.
Rated current = K times sustained maximum load current
where
- K is a factor of safety with respect to the load current and depends upon the nature of the load.
For practically constant loads such as lighting load K = 1.1 – 1.2. If the load current varies considerably as in the case of motors which, during starting periods draw large currents greatly in excess of the normal operating current, the current rating of the fuse element must be selected so as to satisfy the second requirement. This consists in selecting the current rating so that the element will not blow during short-time overload (of 2-10 second duration). It is, therefore, necessary to select an element with such a time-current characteristic that the duration of the overload current will be shorter than the period in which the element will blow at the same value of current. This means that it will carry the short-time overload current without blowing, although its continuous current rating may be less than short-time overload current.
To select the fuse element as mentioned above, its time-current characteristic must be available. As this is not convenient in practice, the element current rating is determined from the following simple expression.
Continuous current rating = Short-time overload current/K
where
- K is the factor of safety with respect to the overload current.
Suitable values for K have been determined on the basis of extensive operating experience and are valid on condition that the overload, and the resultant greater heating of the element due to it, does not lead to oxidation and premature aging of the element.
Depending upon the duty conditions, for overload duration 2-3 seconds (moderate duty) K = 2.5; for overload duration of 10 seconds (severe duty) K = 1.5 – 2. When motors are started frequently the element current rating should be selected by taking the factor for severe duty conditions.
It is noteworthy that a fuse with an element selected keeping in view the short-time overload current will not protect the equipment supplied through it from overload and only serves as a protection against short circuits. Protection against overloads should be provided by separate devices like overload trips.
In houses, blocks of flats, personnel amenity rooms and public buildings, i.e., wherever the electrical circuits are not constantly supervised by trained personnel the fuse elements must satisfy the condition.
Continuous current rating of fuse element = 0.8 x permissible continuous current carrying capacity of the conductors forming the circuits to be protected.
In case of protection for transformers and fluorescent lighting, the selection of fuse should generally have a normal rating 25 to 50 per cent above the normal full-load current of the apparatus to be protected so as to withstand the transient current surge on switching in.
In case of protection of capacitors it is desirable to use a cartridge fuse link having rated current about 25-50% greater than the rated current of capacitors. Protection of capacitors is a difficult task for any protective device due to presence of the higher harmonics in the voltage wave and also due to the fact that the fuses protecting the capacitors are subject to transient currents during switching operation.
Fuses will provide protection against earth faults only when the earth resistance is sufficiently low to enable their operation. In case the earth resistance is high, the fuses will not be able to provide the desired protection and safety.